Update supported versions of Python, Django, Flask

[jwhitlock]

This PR addresses issue #68, updating versions of Python, Django, Flask, ~and Sanic~ to current versions:

• Python 3.7 to 3.10
• Django 3.2 and 4.0
• Flask 0.12, 1.0, 1.1, 2.0, and 2.1
• Sanic 20, ~21, and 22~

Checklist:

• [x] Drop Flask 0.11
• [x] Drop Django 2.2
• [x] Drop Django 3.0
• [x] Add Python 3.10
• [x] Add Django 3.2
• [x] Drop Django 3.1
• [x] Drop Sanic 19
• [x] Drop Python 3.6
• [x] Fix Flask 0.12
• [x] Fix Flask 1.0
• [x] Add Flask 1.1
• [x] Add Flask 2.0
• [x] Add Flask 2.1
• ~Add Sanic 21~ - Larger change, tracked in issue #70
• ~Add Sanic 22~ - Larger change, tracked in issue #70

[codecov-commenter]

Codecov Report

Merging #69 (1d43c9f) into main (5e30d69) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #69   +/-   ##
=======================================
  Coverage   97.81%   97.81%           
=======================================
  Files          19       19           
  Lines         594      594           
  Branches       93       97    +4     
=======================================
  Hits          581      581           
  Misses          9        9           
  Partials        4        4           

Impacted Files	Coverage Δ
src/dockerflow/flask/app.py	94.55% <ø> (ø)

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[jwhitlock]

OK, the changes are ready for review, and the build is back to green.

Additional changes to fix linting:

• Dropped py27 target from black, and re-formatted code
• Fixed *.pyc removal in MANIFEST.in

Additional cleanup:

• Removed some leftover py27 variants from tox.ini
• Removed a skipped Python 2.7 logging test

I added issue #70 to track adding support for newer Sanic versions, which may be tricky.